Android Low Memory Killer Daemon

Introduction

Android Low Memory Killer Daemon (lmkd) is a process monitoring memory state of a running Android system and reacting to high memory pressure by killing the least essential process(es) to keep system performing at acceptable levels.

Background

Historically on Android systems memory monitoring and killing of non-essential processes was handled by a kernel lowmemorykiller driver. Since Linux Kernel 4.12 the lowmemorykiller driver has been removed and instead userspace lmkd daemon performs these tasks.

Android Properties

lmkd can be configured on a particular system using the following Android properties:

• ro.config.low_ram: choose between low-memory vs high-performance device. Default = false.

• ro.lmk.use_minfree_levels: use free memory and file cache thresholds for making decisions when to kill. This mode works the same way kernel lowmemorykiller driver used to work. Default = false

• ro.lmk.low: min oom_adj score for processes eligible to be killed at low vmpressure level. Default = 1001 (disabled)

• ro.lmk.medium: min oom_adj score for processes eligible to be killed at medium vmpressure level. Default = 800 (non-essential processes)

• ro.lmk.critical: min oom_adj score for processes eligible to be killed at critical vmpressure level. Default = 0 (all processes)

• ro.lmk.critical_upgrade: enables upgrade to critical level. Default = false

• ro.lmk.upgrade_pressure: max mem_pressure at which level will be upgraded because system is swapping too much. Default = 100 (disabled)

• ro.lmk.downgrade_pressure: min mem_pressure at which vmpressure event will be ignored because enough free memory is still available. Default = 100 (disabled)

• ro.lmk.kill_heaviest_task: kill heaviest eligible task (best decision) vs. any eligible task (fast decision). Default = false

• ro.lmk.kill_timeout_ms: duration in ms after a kill when no additional kill will be done. Default = 0 (disabled)

• ro.lmk.debug: enable lmkd debug logs, Default = false

• ro.lmk.swap_free_low_percentage: level of free swap as a percentage of the total swap space used as a threshold to consider the system as swap space starved. Default for low-RAM devices = 10, for high-end devices = 20

• ro.lmk.thrashing_limit: number of workingset refaults as a percentage of the file-backed pagecache size used as a threshold to consider system thrashing its pagecache. Default for low-RAM devices = 30, for high-end devices = 100

• ro.lmk.thrashing_limit_decay: thrashing threshold decay expressed as a percentage of the original threshold used to lower the threshold when system does not recover even after a kill. Default for low-RAM devices = 50, for high-end devices = 10

• ro.lmk.psi_partial_stall_ms: partial PSI stall threshold in milliseconds for triggering low memory notification. Default for low-RAM devices = 200, for high-end devices = 70

• ro.lmk.psi_complete_stall_ms: complete PSI stall threshold in milliseconds for triggering critical memory notification. Default = 700

• ro.lmk.pressure_after_kill_min_score: min oom_adj_score score threshold for cycle after kill used to allow blocking of killing critical processes when not enough memory was freed in a kill cycle. Default score = 0.

• ro.lmk.direct_reclaim_threshold_ms: direct reclaim duration threshold in milliseconds to consider the system as stuck in direct reclaim. Default = 0 (disabled)

• ro.lmk.swap_compression_ratio: swap average compression ratio to be used when estimating how much data can be swapped. Setting it to 0 will ignore available memory and assume that configured swap size can be always utilized fully. Default = 1 (no compression).

• ro.lmk.lowmem_min_oom_score: min oom_score_adj level used to select processes to kill when memory is critically low. Setting it to 1001 will prevent any kills for this reason. Min acceptable value is 201 (apps up to perceptible). Default = 701 (all cached apps excluding the last active one).

lmkd will set the following Android properties according to current system configurations:

• sys.lmk.minfree_levels: minfree:oom_adj_score pairs, delimited by comma

• sys.lmk.reportkills: whether or not it supports reporting process kills to clients. Test app should check this property before testing low memory kill notification. Default will be unset.